The 27-year-old, who won the Queens title in 2009, 2011 and 2013, had won his past 19 matches on grass.

"I have only got myself to blame that I lost the first set," the Scot told the ATP website.

"I don't know how many set points I had, but quite a lot of them were on my serve.

"On this surface especially you shouldn't really be losing sets like that. For me, that's what's disappointing really about the match. Then, unfortunately, got broken in the first game of the second set. I couldn't quite get it back."

The 35-year-old Stepanek's reward is a quarterfinal clash with South African Kevin Anderson.

Meanwhile, seven-time Wimbledon champion Roger Federer survived a scare in Halle before booking his place in the quarterfinals with a 6-7 6-4 6-2 win over Joao Sousa.

The Swiss ace, who has won this tournament on six occasions, will face Taiwan's Yen-Hsun Lu for a place in the semifinals.

"It was slightly frustrating at times clearly, but nevertheless I served well and kept on doing my thing," said fourth-ranked Federer, who lost in the last 16 at Roland Garros.

"It was important to stay calm and actually I think it gives me more confidence winning this way.

"I had to stay calm, fight through the match and find a way and then the last set and a half were much better. So, I'm actually pretty happy now."